This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. This is a general investigation of the mechanisms of catalytic hemoproteins and of the relationships between hemoprotein structure and activity. This investigation includes, but is not limited to, studies of the cytochromes P450, horseradish peroxidase, lactoperoxidase, myoglobin, myeloperoxidase, DUOX enzymes, and heme oxygenases. A key element in the study of these proteins is the analysis of product structure, stereochemistry and isotopic substitution. The investigation of protein adducts is also important. In addition to product studies, heavy use is made of mechanism-based and other irreversible inhibitors to characterize the protein active sites. These irreversible inhibitors alkylate either the heme group or the protein. Mass spectrometry is essential for identification of the structures of the heme adducts, and consequently for the use of mechanism-based inhibitors as structural probes. The identification of protein residues that are modified by agents that react with the protein skeleton is also pursued as an additional tool for analysis of the active site structure and mechanism. These investigations, as in the past, are expected to make substantial contributions to our understanding of hemoprotein mechanisms, to the development of potentially useful hemoprotein inhibitors and to our general understanding of protein structure and function.